As part of natural gas processing and hydro-treatment of oil fractions, a large amount of hydrogen sulfide (H2S) is produced. The H2S is toxic and therefore is converted to elemental sulfur (S), which is a more practical and safer state for handling and transportation. With more stringent fuel regulations and increasing environmental concerns, together with the need to process sourer crude oils and natural gases, sulfur recovery has become one of the leading issues in emission reduction. Elemental sulfur is the ultimate state of recovery of the sulfur species.
The conversion of H2S into elemental sulfur is performed in a sulfur recovery unit (SRU). The level of sulfur recovery is increasingly emphasized as the need to minimize the amount of sulfur compounds released to atmosphere from the recovery unit needs to be reduced in order to meet the mandated legal limits. The most common conversion process, for this conversion, is known as the modified Claus treatment process or alternately the Claus unit or modified Claus unit.
The modified Claus treatment process is a combination of thermal and catalytic processes that are used for converting gaseous H2S into elemental sulfur.
Claus unit feed gases have a wide range of compositions. Most of the feed gases originate from absorption processes using various solvents (amine, physical or hybrid solvents) to extract hydrogen sulfide from the by-product gases of petroleum refining, natural gas processing, and also tar sands, coal gasification and other industries. The other gas plant or refinery source of H2S is the sour water stripper unit.
The first process is a thermal process (that is, in the absence of catalyst) in a reaction furnace. The feed gas to the Claus unit is burned in the reaction furnace using sufficient combustion air, or oxygen enriched air to burn a stoichiometric one-third of the contained H2S. The reaction furnace pressure is maintained at about 1.5 bars (35-70 kPa above atmospheric pressure) and the temperature is maintained at about 900-1,350° C. in a “no-preheat” operation case. The H2S from the feed gas is thermally converted into elemental sulfur, along with sulfur dioxide (SO2). Sulfur yield is typically around 65%-72% depending on the operation mode of the SRU. Increasing the elemental sulfur yield in the reaction furnace and subsequently the condenser is advantageous as it reduces the later load on the catalytic reactors. The reaction furnace operation is designed to maximize sulfur recovery in consideration of the feed composition, by adjusting air/oxygen feed, reaction temperature, pressure, and residence time. In addition, the reaction furnace can destroy contaminants, such as hydrocarbons, that are present in the feed gas stream. Such contaminants pose problems for the catalytic reactors through the development of carbon-sulfur compounds that can lead to plugging or deactivation of the catalyst beds.
The hot reaction product gas from the reaction furnace, containing sulfur vapor, can be used to produce high pressure steam in a waste heat boiler, which also results in cooling the product gas. The product gas is then further cooled and condensed in a heat exchanger, while producing additional low pressure steam. The condensed liquid sulfur is separated from the remaining unreacted gas in the outlet end of the condenser and sent to a sulfur pit or other collection area.
The separated gas then enters the catalytic process of the Claus unit. The catalytic process contains between two and three catalytic reactors. Following the sulfur condenser, the separated gas is reheated and enters the first catalytic reactor, which is maintained at an average temperature of about 305° C. In the first catalytic reaction about 20% of the H2S in the feed gas is converted into elemental sulfur through a reaction with the SO2. The temperature is limited by the exit temperature to avoid catalytic bed damages and thermodynamic considerations. The outlet product gas from the first catalytic reactor is cooled, in a second condenser, which can also produce steam. Again, the condensed liquid sulfur is separated from the remaining unreacted gas in the outlet end of the second condenser and sent to sulfur storage. The separated gas from the second condenser is sent to another re-heater and the sequence of gas reheat, catalytic reaction, condensation and separation of liquid sulfur from unreacted gas is repeated for the second and third catalytic reactors at successively lower reactor temperatures. About 5% and 3% of the H2S in the feed gas are converted into elemental sulfur respectively in the second reactor and third reactors.
Finally, the gas stream is released to atmosphere via a stack after passing through an incinerator which oxidizes any remaining sulfur species into SO2. In addition, the flue gas compounds include water, nitrogen, oxygen, sulfide dioxide and eventually carbon dioxide. The eventual presence of carbon dioxide results from the acid gas composition (CO2 and H2S are recovered from natural gas during a sweetening process, such as an amine process). Incinerator temperature and gas temperature in the refractory lined stack are high enough (far above gas dew point) to avoid corrosion and help with quick SO2 dissemination in the surrounding air. Moreover, the stack is designed to make sure SO2 concentration at ground level is below the local regulatory limit.
For a well-designed and well-operated Claus sulfur recovery plant having three catalytic reactors, an overall sulfur conversion of 96-98% can be achieved depending on the feed gas composition. To achieve higher conversion, a tail gas treatment unit must be added to further process the exhaust gas upstream of or as an alternative to an incinerator. Tail gas treatment units are polishing units. Currently available tail gas treatment units can be effective at achieving up to 99.2% recovery, but can add significant capital cost to the Claus treatment unit, often on the same order of magnitude as the Claus unit itself.
Tail gas treatment technologies that have been developed include, but are not limited to, the Scot® process, Highsulf™, BSR/MDEA™, Sultimate™, Bechtel TGTU, and Technip TGTU. The choice of tail gas treatment unit installed depends on the conversion targeted as cost is directly linked to the required conversion level. While the Scot process can reach 99.9% sulfur recovery, the added cost and unit complexity makes this process unfeasible when the Claus feed is not highly concentrated with hydrogen sulfide, e.g., unless greater than 55%. In addition to increase operating and capital costs, these technologies can require significant physical footprint for the various process vessels, columns, pumps, and storage vessels necessary for operation.
Therefore, a process which minimizes SO2 being released to atmosphere without requiring excessive amounts of energy, equipment and materials, or process shutdown is desired. Preferably, such a process, would maintain the overall sulfur capacity of the Claus unit, while increasing the overall sulfur recovery efficiency.